Glacial Sediment Transport
Great strides have been made in recent years in collaborative interpretation of seismic data from the
Antarctic margin (through the ANTOSTRAT initiative: see Cooper et al., 1994; 1995). Together
with the simplicity of the modern Antarctic glacial regime (compared with that of the Arctic), these
data have led to the rapid emergence and application of a unifying model of glacial sediment
transport and deposition (Alley et al., 1989; Larter and Barker, 1989; Bartek et al., 1991; Cooper et
al., 1991; Kuvaas and Kristoffersen, 1991). Briefly, almost all ice transport to the ice-sheet
margins takes place within broad, rapidly moving ice streams. Rapid flow is enabled by low
friction basal conditions, the main source of which is the existence of an overpressured and
undercompacted, unsorted, shearing basal till. The necessary shear ensures that ice transport is
accompanied by till transport, and virtually all of the transported till is melted
out/dropped/deposited very close to the grounding line, where the ice sheet becomes ice shelf
before calving into icebergs and drifting north. The ice stream, therefore, essentially erodes and
transports inshore of the grounding line and deposits directly offshore in a high-latitude analogue
of the low-latitude subaerial erosion/shoreline/marine sedimentation system. Further, the
grounding line advances and retreats under the influence of upstream ice provision and basal
sediment supplyand sea-level changethat are all related to climate. The very large prograded
sediment wedges beneath the Antarctic margin were developed during a series of glacial maxima,
when the ice sheet was grounded all the way to the continental shelf edge (Fig. 5).

The glacial sedimentation regime has other characteristics. Progradation is usually focussed into
broad "trough-mouth fans" opposite the main ice streams, and the shelf is overdeepened (generally
to 300-600 m depth, but in places much deeper) and inward-sloping. Continental slopes are often
steep, and in places turbidity-current transport of the unstable component of slope deposition (with
down-current deposition of suspended fines) has produced large hemipelagic sediment drifts on
the continental rise (Kuvaas and Leitchenkov, 1992; Rebesco et al., 1996; Fig. 6). Sediment supply
to the slope and rise is highly cyclic, with large quantities of unsorted diamicton deposited during
glacial maxima and very little deposited during interglacial periods.

Three depositional environments are recognized: shelf topsets and slope foresets of the prograded
wedge, and proximal hemipelagic drifts on the continental rise. Of these, the shelf record is
potentially the least continuous. There, sediment is preserved mainly as a result of slow subsidence
from cooling and from flexural response to the topset and foreset load, and the sediment is prone
to re-erosion during the next glacial advance. The topsets tend to mark only the major changes in
glacial history, so that the more continuous foreset record is an essential complement. The
proximal rise drifts may not always be present and are as yet sparsely sampled, but potentially
contain an excellent record, closely related to that of the upper slope foresets from which they are
derived. Existing seismic data and drill sites from around Antarctica have demonstrated the coarse
(but not as yet the fine) scale climate record in continental rise sediments and the likely climatic
sensitivity of margin wedge geometry (Barker, 1995), and have revealed the partial nature of the
shelf topset record (Hayes, Frakes, et al., 1975; Barron, Larsen, et al., 1989).

The continental shelf is an area of high biogenic productivity during interglacial periods. Although
long-term sediment preservation on the shelf is limited because of the erosional effects of
grounded ice sheets during subsequent glacials, biogenic interbeds will be preserved within
sequence groups composed mainly of thick glacial diamicton topsets and foresets. In addition,
glacially eroded deeps can preserve expanded Holocene sections that may be continuous and
essentially biogenic, provided the ice-sheet grounding line is sufficiently remote that ice-rafted
debris is minor or absent and the section is sufficiently protected from bottom current action. Such
sections can provide a record of decadal and millennial variability that can be compared with
records from low latitudes and the ice sheet itself. This environment is available on the inner shelf
of the Antarctic Peninsula (Domack and McClennen, 1996) and will be sampled during Leg 178.

Regional Features of Antarctic Glaciation
Different parts of Antarctica have had different glacial histories. The present Antarctic ice sheet
comprises an East Antarctic component grounded largely above present sea level and a West
Antarctic component grounded largely below sea level. Marine-based (West Antarctic) ice sheets
are considered less stable. There is evidence from around Antarctica that, although East and West
Antarctic climates were coupled in the past, changing approximately in phase, the climate of West
Antarctica (including the Antarctic Peninsula) has varied around a consistently warmer baseline.
Although East Antarctic glaciation extends to 35 Ma or earlier, West Antarctic glaciation probably
began more recently, during generally colder times. Further, there is strong evidence that Northern
Hemisphere glaciation has been the main contributor to global sea-level change over the past 0.8
m.y. and probably 2.5 m.y., and has therefore partially driven the more subdued changes in
Antarctic glaciation. Another significant local control may have been the Transantarctic Mountains,
which probably attained much of their present elevation and influence on the East Antarctic ice
sheet during late Cenozoic time.

Antarctic Peninsula RegionTectonic Influences On Sedimentation
The tectonic setting of the Antarctic Peninsula is unusual, but straightforward. Subduction of the
Pacific ocean floor that had occurred for 150 m.y. or more ended with collision of a (Phoenix
Antarctic) ridge crest at the trench, earliest (~50 Ma) in the southwest and latest (6-3 Ma) in the
northeast. In the far northeast, the surviving South Shetland Trench and extensional Bransfield
Strait form a modern complexity that does not concern us here. Generally, the effects of collision
have included (1) some terrigenous sedimentation in and beyond the ridge crest in the last 2-3
m.y. before collision and (2) uplift of the margin soon after collision followed by slow subsidence,
leading to a hiatus in terrigenous sediment supply to the rise in that particular collision segment for
a few million years after collision. Collisions occurred well before the onset of glaciation in the
southwest, but not in the northeast. In the northeast, this provides a useful constraint on the
maximum age of glacial sediments (they overlie ocean floor of known age), but also threatens
interference between tectonic and glacial events. For the older glacial history it is prudent to avoid
the northeast area of the margin.
Antarctic Peninsula Glacial Sedimentation
The ultimate aim of the four or five linked ANTOSTRAT drilling proposals is to provide an
estimate of the variation in size of the Antarctic Ice Sheet through the Cenozoic. Each
ANTOSTRAT proposal is focussed on the particular contributions its region might make toward
understanding Antarctic glacial history. A single region does not offer the best opportunities for
drilling in all respects. The particular value of drilling on the Antarctic Peninsula is made clear
below, in terms of the main influences on glacial sedimentation.

1. All Antarctic margins are extensional or effectively so, in a thermal and flexural
sense, but most are old. Age governs thermal subsidence and rigidity, which
controls response to erosion and deposition and to cyclic ice loading. The Antarctic
Peninsula behaves as a young passive margin, having subducted a ridge crest (50
Ma in the southwest to only 6-3 Ma in the northeast; Barker, 1982; Larter and
Barker, 1991a). The margin undergoes steady thermal subsidence, which means
better preservation of topset beds of the prograded wedge than at an older, colder
margin, and a more local isostatic response to sediment load.

2. Snow accumulation varies with temperature and is greatest around the continental
edge and particularly along the Antarctic Peninsula, which is warmer than East
Antarctica (Drewry and Morris, 1992). Snow accumulation governs the required
rates of ice transport, hence basal sediment transport. Greater accumulation means
an expanded sediment record. Warmer ice means (probably) faster ice flow, which
also contributes to a rapid response to climate and an expanded sediment record.

3. The extent of the ice drainage basin affects the speed of response to climate change
and adds the complexity of a distal to a proximal signal (which allows the
possibility of seeing the effects of a small, purely inland ice sheet at the coast during
less-glaciated periods). The Antarctic Peninsula is a narrow strip of interior upland,
dissected by fjords and bordered by a broad continental shelf. It therefore has a
low-reservoir, high-throughput glacial regime with only a proximal source, so it is
both simple and highly responsive to climate change.

4. Subice geology (resistance to erosion) is a significant variable, to the extent that a
till base facilitates ice streaming. The Peninsula interior is 2000 m high, composed
largely of Andean-type plutonic and volcanic rocks. Before ridge subduction, the
Pacific margin was a well-developed forearc terrain on which the glacial regime has
superposed an extensive prograded wedge (Larter and Barker, 1989, 1991b;
Anderson et al., 1990; Larter and Cunningham, 1993; Bart and Anderson, 1995).
The topography and geology of the Peninsula vary very little along strike, which
simplifies models of erosional and depositional response to climate change. Short
cores on the outer shelf show diamicton beneath a thin cover of Holocene
hemipelagic mud (Pope and Anderson, 1992; Pudsey et al., 1994).

Onshore evidence of Eocene glaciation on the South Shetland Islands (northern Antarctic
Peninsula) has been published (see Birkenmajer, 1992), but this conflicts with other evidence of
regional climate. Generally, it is considered that the Antarctic Peninsula can provide a high
resolution record of glaciation back to perhaps 10 Ma. To go back farther could involve
entanglement with the tectonics of ridge-crest collision, making this a problem rather than an asset.
However, because of the Antarctic Peninsula's more northerly position, its glacial history is shorter
than East Antarctica's. The record before 10 Ma may be largely nonglacial, or may reveal a stage
of valley glaciation lacking regular ice-sheet extension to the continental shelf edge.